Selective Diffusive Gradients in Thin Films (DGT) for the Simultaneous Assessment of Labile Sr and Pb Concentrations and Isotope Ratios in Soils

A method using diffusive gradients in thin films (DGT) for the accurate quantification of trace-level (μg L–1) Sr and Pb concentrations and isotope ratios [δSRM 987(87Sr/86Sr) and δSRM 981(207Pb/206Pb)] in labile, bioavailable element fractions in soils is reported. The method is based on a novel poly(tetrafluoroethylene) (PTFE) membrane binding layer with combined di(2-ethyl-hexyl)phosphoric acid (HDEHP) and 4,4′(5′)-bis-t-butylcyclohexano-18-crown-6 (crown-ether) functionality with high selectivity for Sr and Pb (TK100 membrane). Laboratory evaluation of the TK100 DGT showed linear uptake of Sr over time (2–24 h) up to very high Sr mass loadings on TK100 membranes (288 μg cm–2) and effective performance in the range of pH (3.9–8.2), ionic strength (0.001–0.1 mol L–1), and cation competition (50–160 mg L–1 Ca in a synthetic soil solution matrix) of environmental interest. Selective three-step elution of TK100 membranes using hydrochloric acid allowed us to obtain purified Sr and Pb fractions with adequate (≥75%) recovery and quantitative (≥96%) matrix reduction. Neither DGT-based sampling itself nor selective elution or mass loading effects caused significant isotopic fractionation. Application of TK100 DGT in natural soils and comparison with conventional approaches of bioavailability assessment demonstrated the method’s unique capability to obtain information on Sr and Pb resupply dynamics and isotopic variations with low combined uncertainty within a single sampling step.


Experimental Section
S-3 Laboratory procedures and materials S-3 Table S1. Instrumental setups and parameters for Sr and Pb isotope ratio measurements S-4 Figure S1. Uptake efficiency of TK100 resin and gels for Sr S-5 Uncertainty estimation S-6 Diffusion cell experiment S-7 Solution preparation in pH and ionic strength experiments S-8 Solution preparation and analysis in time-dependent accumulation and isotopic fractionation experiments S-9 Table S2. Composition of synthetic soil solutions S-10 DGT deployment, soil solution sampling, and NH4NO3 extraction S-11 Results and Discussion S-12 Table S3. Applied elution schemes and elution recoveries S-12 Table S4. Diffusion coefficients of Sr and Pb in APA diffusive gel S-13 Table S5. DGT-measured solution concentrations (cDGT), bulk solution concentrations (csoln), and ratios of cDGT to csoln values of Pb in synthetic soil solutions S-14 Table S6. Efficiency of TK100 DGT for separation of Sr and Pb from different potentially interfering matrix cations S-15 Table S7. Elemental mass concentrations in soil solutions of the experimental soils S-16 Table S8. Isotope ratios of Sr and Pb in NH4NO3 (c = 1 mol L -1 ) extracts of the experimental soils S-17 S-3

Experimental Section
To comply with notation conventions used in DGT literature, the symbol c is used throughout this work for both molar and mass concentrations in solution instead of the designated symbols c and γ recommended by the International Union of Pure and Applied Chemistry (IUPAC).  S-8

Solution preparation and analysis in pH and ionic strength experiments
To test the effect of pH on TK100 DGT performance for simultaneous sampling of Sr and Pb, TK100 DGT devices were deployed in solutions with pH 3.9, 4.8, 6.5, 7.5, and 8.2 at a constant NaNO3 (c = 0.01 mol L -1 ) background. The pH was adjusted using dilute HNO3 or NaOH (both c = 1 mol L -1 ).

DGT deployment, soil solution sampling, and NH4NO3 extraction
For DGT deployment, ~150 g of each soil sample was mixed with laboratory water type 1 to reach 100 % of their maximum water holding capacity. The saturated soil pastes were incubated at 24 °C for 24 h and then applied as a ~2 mm-thick layer on the DGT devices (n = 3). The DGTs were then gently tapped a few times to ensure a complete contact between the filter surface of the sampler and the soil. DGT sampling was conducted in closed plastic boxes for 24 h at 24 °C in an incubator. After deployment, DGTs were washed to remove the adhering soil paste and disassembled. The TK100 discs were retrieved and subjected to three-step elution. Sub-samples (V = ~20 mL) of the incubated soil pastes were used to collect soil solution by centrifugation at 5000 × g for 15 min. 14 The supernatants were passed through a 0.45 µm-pore size PES syringe filter, immediately acidified to give a dilute HCl (w = 2 %) matrix, and then analyzed by ICP-MS. In parallel, NH4NO3 (c = 1 mol L -1 ) extraction of soil samples was performed according to DIN ISO 19730. 15 The NH4NO3 extracts were diluted 20-fold and subjected to matrix separation using an automated, low-pressure chromatographic system (prepFAST-MC, ESI).
Simultaneous separation of Sr and Pb was achieved using self-packed columns with 3 mL bed volume of DGA resin (TrisKem). Recoveries over all samples were ≥98 % for Sr and ≥71 % for Pb. A detailed description of the procedure can be found elsewhere. 16 S-12  (1) and (3), all others are 1SD (n = 4).   S-17